1. Field of the Invention
The invention relates to a control apparatus and a control method for an internal combustion engine.
2. Description of Related Art
A fuel cut control in which fuel injection and ignition are temporarily stopped at the time of deceleration or the like is known. In some cases, the fuel cut may restrain unnecessary consumption of fuel. During execution of the fuel cut, the air/fuel ratio inevitably becomes lean. This makes it possible to diagnose the presence or absence of abnormality of an air/fuel ratio sensor, or the like, on the basis of whether during execution of the fuel cut, the output of the air/fuel ratio sensor shows that the air/fuel ratio is lean.
If the fuel cut is executed, for example, when a catalyst has high temperature, particles of platinum (Pt) constituting the catalyst may react with oxygen, which is supplied in excess, and may grow into large particles of Pt. This can result in a reduced total surface area of Pt, that is, a reduced contact area of Pt with exhaust gas. Consequently, the emissions purifying performance of the catalyst may decline. As a measure to cope with this phenomenon, it is possible to perform a catalyst degradation restraint process of prohibiting the fuel cut when the temperature of the catalyst is higher than a threshold value.
If the fuel cut is prohibited, the reduction in the number or rate of opportunities of execution of the fuel cut is accompanied by a reduced number or rate of opportunities to diagnose the presence or absence of abnormality of the air/fuel ratio sensor. As a measure to cope with this phenomenon, Japanese Patent Application Publication No. 2009-167937 (JP 2009-167937 A) describes a technology in which execution of the catalyst degradation restraint process is prevented when the presence or absence of abnormality of the air/fuel ratio sensor has not been determined.
In the case where the catalyst temperature is high and the catalyst degradation restraint process is restricted because the presence or absence of abnormality of the air/fuel ratio sensor has not been determined, if determination of the presence or absence of abnormality of the air/fuel ratio sensor is completed during execution of the fuel cut, the fuel cut may be prohibited and therefore fuel injection and ignition can be started. If the catalyst temperature declines after that, the fuel cut may be executed again. Intermittent repetitions of execution of the fuel cut in the foregoing manner may intermittently cause vibration and noise of the internal combustion engine. As a result, discomfort or sense of strangeness may be caused to an occupant of the vehicle.